1. Chapter 10 How Cells Divide

2. 10.1 Prokaryotes vs Eukaryotes Prokaryotes divide by binary fission (simple dividing into two equal halves) DNA copy is made (22 different proteins involved) New plasma membrane grows between genomes New cell wall forms around the membrane Eukaryotes possess chromosomes and divide by mitosis Chromosomes were observed by Walther Fleming in 1882 Chromosomes number can vary greatly from one species to another. Ex: humans have 46; penicillium (fungus) has 1 pair 10.2 Chromosomes are Highly Ordered Structures Chromosome structure-made of chromatin (DNA & protein; 40% DNA and 60% protein) Typical human chromosome contains about 140 million nucleotides Every 200 nucleotides the DNA duplex is coiled around a core of eight histone proteins forming a nucleosome.

3. 10.2 cont’d Heterochromatin-highly condensed (usually permanently condensed) Euchromatin-condensed only during cell division (expressed portion of DNA) Karyotype-an array of chromosomes

4. 10.2 cont’d How many chromosomes are in each cell? Every cell with the exception of gametes and a few specialized tissues is diploid (2n). The haploid gametes (1n) contain only one copy of each type of chromosome. The two copies of each chromosome are called homologous chromosomes. Each homologue replicates before division, producing two sister chromatids. They are joined together by a centromere.

5. 10.3 The Cell Cycle Five phases-Interphase (3), Mitosis and Cytokinesis Animation: Mitosis and Cytokinesis Animation: Mitosis and Cytokinesis

6. G1(Gap) Cell growth & development S phase (synthesis) DNA is replicated; must be exact G 2 (Gap) Replication of organelles Prophase Sister chromatids tightly coil (longest phase) Metaphase Sister chromatids line up along center of cell (shortest phase) Telophase Chromosomes move to opposite poles Anaphase Chromosomes split Cytokinesis Division of cytoplasm Interphase- “resting phase. DNA is known as chromatin (long, thin and relaxed) Sister chromatids- original & copy Chromosomes are pulled apart by spindle fibers (housed by centrioles) MITOSIS

7. 10.6 Cell Cycle Controls Cell Cycle Control-the cycle has checkpoints to allow sufficient time for all events to occur. 1.G 1 Checkpoint-located near G 1 -decides whether cell should divide, delay division or enter the resting phase (G 0 ) Production of cyclins (proteins) can trigger the next round of DNA replication and division. Cyclins associate with cyclin-dependent kinases and activate particular cellular processes. 2.G 2 Checkpoint-occurs at the end of G 2 -triggers the start of mitosis. Cell accumulates G 2 cyclin which binds to a CDK to form a complex called the mitosis promoting factor. When MPF level exceeds a threshold necessary to trigger mitosis, G 2 ends. 3. M Checkpoint-occurs at metaphase-triggers the exit from mitosis & cytokinesis and the beginning of G 1.

8. G 1 (Gap) Cell growth & development S phase (synthesis) DNA is replicated; must be exact G 2 (Gap) Replication of organelles Prophase Sister chromatids tightly coil (longest phase) Metaphase Sister chromatids line up along center of cell (shortest phase) Telophase Chromosomes move to opposite poles Anaphase Chromosomes split Cytokinesis Division of cytoplasm Interphase- “resting phase. DNA is known as chromatin (long, thin and relaxed) Sister chromatids- original & copy Chromosomes are pulled apart by spindle fibers (housed by centrioles) MITOSIS G 1 checkpoint G 2 checkpoint M checkpoint

9. 10.6 cont’d Multicellular Control Cells will stop dividing when they come in contact with each other. When there is a need and there is room, cells bind small amounts of growth factors (proteins that stimulate cell division). Ex: platelet derived growth factor-platelets in blood clots release growth factors that override cellular control. This helps heal wounds. Over 50 different proteins that function as growth factors have been isolated. Specificities vary. Cancer & Proliferation p53 gene plays a role in G 1 checkpoint p53 protein monitors the integrity of DNA. If DNA is damaged, p53 halts division so DNA can be repaired. If damage cannot be repaired, p53 directs cell to kill itself (apoptosis)

10. 10.6 cont’d If p53 is damaged, cell could divide with damaged DNA. Proto-oncogenes-genes that normally stimulate cell division. Mutations in them can cause them to be overexpressed and hyperactive. They are converted into oncogenes-this lends to excessive cell proliferation. (30 proto-oncogenes known) Tumor suppressor genes-genes that normally inhibit cell division. When mutated they can lead to unrestrained cell division. Most thoroughly understood is the Rb gene.